Imre Holb

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

Yield Loss in Apple Caused by Monilinia fructigena (Aderh. & Ruhl.) Honey, and Spatio-temporal Dynamics of Disease Development

Monilinia fructigena (Aderh. & Ruhl.) Honey causes considerable yield losses in pome fruit culture. During a field study in the Netherlands in 1997 and 1998, the increase in disease incidence in time was assessed and final pre- and post-harvest losses were recorded in the susceptible apple cultivars James Grieve and Coxs Orange Pippin. Each individual tree was considered as a unique quadrat, and the spatial distribution of diseased fruits among fruit trees at every assessment date was characterised by a dispersion index, Lloyds index of patchiness (LIP). Spatial autocorrelation was applied to detect potential clustering of trees with diseased fruits within rows. In cv. James Grieve, the rate of increase of disease incidence was constant up to harvest time, whereas in cv. Coxs Orange Pippin disease incidence increased markedly 3 weeks before harvest time, which coincided with the harvest of cv. James Grieve in neighbouring rows. Pre-harvest disease incidence was 4.2–4.3% in cv. James Grieve in both years, in cv. Coxs Orange Pippin this was 4.4% in 1997 and 2.7% in 1998. Post-harvest yield losses amounted on average 1.5–2.0% for both cultivars, no significant differences were found between the cultivars (t-test, P=0.05). Both in 1997 and 1998, clustering of diseased fruits among fruit trees was detected; LIP values were significantly higher than 1 (P=0.05 in 1997, P=0.01 in 1998). Clustering of trees with diseased fruits was detected in 1998, when significant (P=0.05) positive correlation coefficients occurred for 2nd, 3rd and 4th lag-order distances in cv. James Grieve, and a significant (P=0.05) positive first-order correlation in cv. Coxs Orange Pippin. Wounding agents, such as insects and birds, may play an important role in the underlying disease dynamics, and crop losses may be minimised by control of these agents.

Overwintering of Conidia of Venturia inaequalis and the Contribution to Early Epidemics of Apple Scab

Overwintering of conidia of Venturia inaequalis associated with shoots and buds was determined, and the contribution to early spring epidemics of apple scab was evaluated during three consecutive seasons (1999 to 2001) in the Netherlands. Examinations of shoot samples collected before bud break showed that the percentage of shoots with superficial black fungal mycelia or conidia was above 65%, and the mean number of conidia on a 1-cm piece of shoot length ranged from 581 to 1,033. However, germination tests showed that the viability of conidia on shoots was less than 1.5%. No macroscopic scab lesions were detected on the scales of dormant buds. However, microscopic examinations of individual bud tissues demonstrated that the number of conidia was >3,000 per 100 buds in each year. The mean viability of conidia associated with buds ranged from 0.7 to 1.9% and from 3.7 to 10.5% for the outer and inner bud tissues, respectively. Results of field assessments at tight-cluster phenological stage showed that the percentage of infection caused by the viable overwintered conidia ranged from 0.3 to 3.8% in the various treatments. Our results indicated that conidia were unlikely to overwinter on the surface of shoots or outer bud tissues, where they were exposed to fluctuating environmental conditions, and, consequently, were unlikely to play a role in initiating an early epidemic of apple scab in the spring. However, our results indicated a risk from overwintered conidia in the inner bud tissues arising from a high level of scab the previous autumn. Therefore, orchards with high levels of apple scab, where ascosporic inoculum is much reduced, e.g., by sanitation, should be protected in early spring by means of fungicide treatment at green tip.

Effect of six sanitation treatments on leaf litter density, ascospore production of Venturia inaequalis and scab incidence in integrated and organic apple orchards

A two-year study was conducted to determine the effect of six sanitation treatments on leaf litter density (LLD), relative ascospore production of Venturia inaequalis and scab incidence on spur-leaf clusters, leaves and harvested fruits, on two cultivars with low and high scab susceptibilities, in Hungarian integrated and organic apple orchards. The following sanitation treatments were used: sprays of lime sulphur in autumn, collecting fallen leaves in autumn, straw mulch cover in late winter, sprays of lime sulphur followed by mulch cover, collecting fallen leaves followed by mulch cover, collecting fallen leaves followed by covering the orchard floor with plastic foil, and non-sanitized control. LLD decreased continuously in all treatment plots by 0–23% by mid-May in both orchards and years; however, LLD reduction was 1.4–4.2 times higher in the organic orchard compared to the integrated one. All treatments, except for the lime sulphur treatment, resulted in significant (P < 0.05) reduction of LLD and ascospore production in both the integrated and organic apple orchards compared to non-sanitized plots. The most efficient treatment was leaf collection combined with plastic foil cover, followed by leaf collection combined with mulch cover, leaf collection alone, mulch cover alone, and lime sulphur spray combined with mulch cover, with a reduction in the ascospore production of >95, 72–92, 56–79, 24–38, and 27–46%, respectively, in the mean of both orchards and years. However, only treatments of leaf collection applied alone, or in combination with mulch or with plastic foil cover reduced significantly (P < 0.05) leaf and/or fruit scab incidence by 18–57% compared to non-sanitized plots. These three leaf collection treatments are recommended in both integrated and organic orchards and the possibilities of successfully incorporating these methods into orchard management practices are interpreted.

The widespread occurrence of overwintered conidial inoculum of Venturia inaequalis on shoots and buds in organic and integrated apple orchards across the Netherlands

A 2-year study was conducted to determine the widespread occurrence of overwintered conidial inoculum of Venturia inaequalis and its impact on the apple scab control in 18 apple orchards (organic and integrated) with various levels of scab in the Netherlands. Autumn assessments of scab lesions showed that the integrated orchards had a significantly lower scab incidence (<20%) compared to that of the organic orchards (>60%). At the bud-break phenological stage, the mean numbers of nonviable and viable conidia on 1 cm pieces of shoots ranged from 1 to about 90 and from 6 to more than 1000 in the integrated and the organic orchards, respectively, for both years. However, viable conidia on shoots were found only in 2 integrated and 6 organic orchards out of the 18 and the viability of conidia was below 2%. The mean numbers of viable and nonviable conidia per 100 buds ranged from 24 to more than 1000 and from 230 to almost 5000 in the integrated and the organic orchards, respectively, for both years. In both years, some 60–85% of the conidia was found on the outer bud scales. The percentage viability associated with the outer bud tissues was below 2% for all the orchards. However, the percentage of viable conidia within the inner bud tissues ranged from 0% to 6% in the integrated and from 2% to 11% in the organic orchards for both years. Differences between the organic and the integrated orchards were clearly demonstrated for overwintered conidia associated with both shoot and bud samples. The relationship between autumn scab incidence and numbers of overwintered conidia associated with shoots or buds was exponential. If the autumn scab incidence was above 40%, then the number of overwintered conidia markedly increased. We conclude that specific treatments for overwintering conidia of Venturia inaequalis may not be necessary in integrated orchards with a low scab incidence in the previous autumn. However, the risk of early scab epidemics initiated by overwintered conidia potentially is high in organic orchards. Preventative measures in early spring and also in the previous year must be established in these orchards.

Effect of Fungicide Treatments and Sanitation Practices on Brown Rot Blossom Blight Incidence, Phytotoxicity, and Yield for Organic Sour Cherry Production

Demand for organically grown fruit, including sour cherry, is rising in Europe and the United States, but the limited tools for disease management have not been thoroughly investigated. In this study, management of brown rot blossom blight, caused by Monilinia laxa, was examined for organic sour cherry production in Hungary. Combinations of sanitation practices and fungicide treatments, including copper hydroxide, lime sulfur, and micronized and nonmicronized wettable sulfur, were investigated in 2 years and two cultivars. The effect of fungicide treatments on yield and phytotoxicity on spur-leaf clusters was also determined. Among fungicide treatments suitable for organic production, copper hydroxide and lime sulfur alone or in combination with micronized wettable sulfur were most effective for blossom blight control when applied twice (at closed blossom and full bloom) or three times (at closed blossom, full bloom, and petal fall) during bloom. Both treatments were not as effective as the conventional standard and caused more damage on spur-leaf clusters during wet weather conditions, but significantly increased crop yield compared with the untreated control or wettable sulfur treatments. Micronized and nonmicronized sulfur applied up to three times during bloom were equally effective, did not impact yield, were not phytotoxic, and reduced blossom blight compared with the untreated control. Sanitation (the removal of blighted twigs and mummified fruit) reduced blossom blight in both cultivars compared with nonsanitized plots when disease pressure was high. The need for an integrated approach to effectively manage blossom blight in organic sour cherry production is discussed. This is the first in-depth characterization of cultural and chemical brown rot blossom blight control options for organic sour cherry production.

Effect of pruning on apple scab in organic apple production

In a 3-year Hungarian study, the effectiveness of apple tree pruning as a management tactic for control of apple scab (Venturia inaequalis) was investigated on two susceptible (cvs. Jonagold and Mutsu), two moderately susceptible (cvs. Elstar and Idared), and two Vf resistant apple cultivars (cvs. Liberty and Prima) in three high-density organic apple orchards treated with copperand sulfur-based fungicide programs. The area under the disease progress curve (AUDPC) for leaf and fruit incidences was calculated to evaluate three winter pruning treatments (unpruned, weakly pruned, and strongly pruned). Strong pruning significantly decreased leaf scab on the susceptible and the moderately susceptible cultivars at all sites compared with unpruned ones. Moreover, for the cultivar Jonagold in all years and for the cultivar Mutsu in 2001, AUDPC values of strongly pruned trees were significantly lower at P = 0.05 compared with the weaklypruned trees. The effect of pruning on development of fruit scab was weaker than on leaf scab in all years and at all sites. For fruit scab incidence on susceptible cultivars, only strong pruning treatments decreased AUDPC significantly compared with unpruned treatments. The effects of pruning treatments on moderately susceptible cultivars were significant only in 2001 at all sites. Resistant cultivars showed no significant effect of pruning treatments on development of either fruit or leaf scab except for cultivar Liberty in 2001 at one site. Pruning resulted in no consistent difference in the apple tree canopy microclimate. However, strong pruning resulted in improved spray deposition in the tree canopy when applications were made with an airblast sprayer at midsummer. Results are compared with similar studies, and their biological interpretation is discussed.

Fungal Disease Management in Environmentally Friendly Apple Production – A Review

Many pesticides are used very effectively against fungal diseases in crop protection. However, the widespread use of synthetic pesticides in conventional fruit production clearly indicates that pesticides have several limitations and serious harmful effects on the environment and on human health. This prompted a serious need for a more environmentally benign view in the practice of fruit growing and particularly in plant protection, which also strengthened the concept of environment-friendly approach for apple. In this review article, the present status, possibilities and approaches towards fungal disease management for organic and integrated apple production systems, which are the most prominent environmentally friendly production systems of apple, are reviewed. The review focuses on the control of five important apple diseases: apple scab (Venturia inaequalis), apple powdery mildew (Podosphaera leucotricha), European canker (Nectria galligena), brown rot (Monilinia spp.) and the disease complex of flyspeck and sooty blotch. The first section of this study provides background information and basic features of current disease control in both apple production systems. Then, in the second section of this study, details of novel aspects of non-chemical control approaches against apple fungal diseases, including agronomic measures, mechanical, physical and biological control options as well as essential features of apple cultivar resistance to fungal diseases are given. The overview on five groups of agronomic measures: (1) cropping system and cover crop, (2) plant material and planting, (3) pruning and canopy management, (4) orchard floor management and (5) nutrient supply and harvest, and another five groups of mechanical and physical control methods: (1) pruning, (2) removal of inoculum sources, (3) shredding of leaf litter, (4) burying of inoculum sources and (5) flaming of leaf litter, showed that these non-chemical control measures are one of the most essential approaches for reducing the infection potential of inoculum sources in apple orchards. However, most of these methods are not widely spread in the apple-growing practice due to their high labour costs and/or time limits during the season. We showed that expert-system-based automatisation in the future may greatly enhance the effective integration of these methods into apple growing. We also described almost 30 biological control options, including antagonists, extracts/oils of plants and composts, which were explored recently against fungal diseases of apple, though only few of them are commercially available for the apple-growing practice. Most of these biological control options are suitable only for organic apple growing, as their effectiveness against the key fungal diseases is not able to fulfil the requirements for integrated apple orchards or they are not substantially cost-effective. Developing an effective biological control against polycyclic fungal diseases of apple will be a great challenge in the future for preharvest disease management programmes. In our literature analyses, host resistance, based on breeding programmes for multiple disease resistance, was evaluated as the greatest potential in the effective disease management of environmentally friendly apple production systems. Theoretically, aiming for complete host disease resistance would result in eliminating one of the basic elements of the epidemic triangle and omission of chemical control approaches from disease management of apple.

Characterization of Three-Dimensional Spatial Aggregation and Association Patterns of Brown Rot Symptoms within Intensively Mapped Sour Cherry Trees

BACKGROUND AND AIMS Characterization of spatial patterns of plant disease can provide insights into important epidemiological processes such as sources of inoculum, mechanisms of dissemination, and reproductive strategies of the pathogen population. Whilst two-dimensional patterns of disease (among plants within fields) have been studied extensively, there is limited information on three-dimensional patterns within individual plant canopies. Reported here are the detailed mapping of different symptom types of brown rot (caused by Monilinia laxa) in individual sour cherry tree (Prunus cerasus) canopies, and the application of spatial statistics to the resulting data points to determine patterns of symptom aggregation and association. METHODS A magnetic digitizer was utilized to create detailed three-dimensional maps of three symptom types (blossom blight, shoot blight and twig canker) in eight sour cherry tree canopies during the green fruit stage of development. The resulting point patterns were analysed for aggregation (within a given symptom type) and pairwise association (between symptom types) using a three-dimensional extension of nearest-neighbour analysis. KEY RESULTS Symptoms of M. laxa infection were generally aggregated within the canopy volume, but there was no consistent pattern for one symptom type to be more or less aggregated than the other. Analysis of spatial association among symptom types indicated that previous years twig cankers may play an important role in influencing the spatial pattern of current years symptoms. This observation provides quantitative support for the epidemiological role of twig cankers as sources of primary inoculum within the tree. CONCLUSIONS Presented here is a new approach to quantify spatial patterns of plant disease in complex fruit tree canopies using point pattern analysis. This work provides a framework for quantitative analysis of three-dimensional spatial patterns within the finite tree canopy, applicable to many fields of research.

Monitoring conidial density of Monilinia fructigena in the air in relation to brown rot development in integrated and organic apple orchards.

In a three-year Hungarian study, conidial density of Monilinia fructigena in the air determined from mid-May until harvest was related to brown rot disease progress in integrated and organic apple orchards. Conidia of M. fructigena were first trapped in late May in both orchards in all years. Number of conidial density greatly increased after the appearance of first infected fruit, from early July in the organic and from early August in the integrated orchard. Conidial number continuously increased until harvest in both orchards. Final brown rot incidence reached 4.3–6.6% and 19.8–24.5% in the integrated and organic orchards, respectively. Disease incidence showed a significant relationship with corresponding cumulative numbers of trapped conidia both in integrated and organic orchards, and was described by separate three-parameter Gompertz functions for the two orchards. Time series analyses, using autoregressive integrated moving average (ARIMA) models, revealed that the temporal patterns of the number of airborne conidia was similar in all years in both integrated and organic orchards. Conidia caught over a 24-h period showed distinct diurnal periodicity, with peak spore density occurring in the afternoon between 13.00 and 18.00. Percent viability of M. fructigena conidia ranged from 48.8 to 70.1% with lower viability in dry compared to wet days in both orchards and all years. Temperature and relative humidity correlated best with mean hourly conidial catches in both integrated and organic apple orchards in each year. Correlations between aerial spore density and wind speed were significant only in the organic orchard over the 3-year period. Mean hourly rainfall was negatively but poorly correlated with mean hourly conidial catches. Results were compared and discussed with previous observations.